The Kuril Basin and the Kuril-Kamchatka Trench are two interconnected deep-sea ecosystems both located in one of the most highly productive regions of the world’s oceans. The main distinguishing features of these deep-sea ecosystems are the low oxygen concentration in the near-bottom water in the Kuril Basin, and the high hydrostatic pressure in the trench. We investigated the abundance of meio- and macrobenthic nematodes and macrofauna on the Kuril Basin floor (depths of 3,300–3,366 m) and in the Kuril-Kamchatka Trench area (depths of 3,432–9,539 m), as well as the influence of some environmental factors on the quantitative distribution of bottom fauna. This was not studied so far. The study also focused on the species composition and quantitative distribution of Polychaeta and Bivalvia, which were dominant in abundance among macrofaunal samples. The main factors influencing the quantitative distribution of macrofauna and nematodes were depth, oxygen concentration, and structure of bottom sediments. The Kuril Basin bottom communities are characterized by a high abundance of nematodes and macrofauna, a high species richness of polychaetes, and a pronounced dominance of small-sized species of Polychaeta and Bivalvia, which are probably more tolerant to low oxygen concentrations. Compared to the Kuril Basin, the Kuril-Kamchatka Trench area (at depths of 3,432–5,741 m) had a more diverse and abundant macrofauna, and a very high abundance of meio- and macrobenthic nematodes. In the trench (at depths more than 6,000 m), the diversity of macrofauna and the abundance of macrobenthic nematodes decreased, while the abundance of macrofauna increased with increasing depth. On the trench floor, the macrofaunal abundance was highest due to the high density of populations of several bivalve and polychaete species, apparently adapted to the high hydrostatic pressure on the trench floor. Obviously, the high primary production of surface waters supports the diverse and abundant deep-sea bottom fauna in the studied areas of the northwestern Pacific. Furthermore, a large number of animals with chemosynthetic endosymbiotic bacteria were found in the bottom communities of the Kuril Basin and the Kuril-Kamchatka Trench. This suggests a significant contribution of chemosynthetic organic carbon to functioning of these deep-sea ecosystems.
The abyss of the Sea of Japan represents an example of an isolated deep-sea environment that contains mostly endemic fauna and has a complicated Quaternary history. To determine the trophic structure and sources supporting this abyssal benthic community, the carbon and nitrogen stable isotope ratios and fatty acid (FA) compositions of key invertebrate species and sedimentary organic matter (SOM) were analysed. Samples were collected at a range of depths from 2481 to 3666 m in the deep-water basin of the Sea of Japan in August 2010. Species of the most abundant invertebrates -including polychaetes, sea anemones, peracarid crustaceans, bivalves and brittle stars -showed similar δ 15 N values, corresponding to relatively high and similar trophic positions. Analysis of FA trophic markers showed that all of these equally 15 N-enriched omnivores, carnivores and scavenger species in the Sea of Japan abyssal environment fed mostly on sinking zooplankton animals. The resulting FA profiles of these species showed a high 18:1ω9/18:1ω7 ratio, and the 22:6ω3 and 20:5ω3 polyunsaturated FAs were the most abundant FAs. Only one macrobenthic species, the filter-feeding Thyasira (Parathyasira) sp., had low 22:6ω3 and 20:5ω3 FA proportions, but it exhibited significant levels of 18:2ω6 and 16:1ω10 FAs characteristic of the SOM of the deep waters of the Sea of Japan. These data reveal the dominant role of descending zooplankton as a food resource for mega-and macrobenthos in this marginal deep-water environment. Despite the proximity of the productive shelf area, which exports plant residues to the deep-water basin of the Sea of Japan, we found no isotopic or FA indications of feeding on allochtonous detritus of seagrasses or macroalgae among abyssal consumers. Our data did not support cannibalism as a feeding mode of the abundant abyssal carnivorous polychaetes, as had previously been suggested. Different key invertebrate species of the Sea of Japan abyssal food web occupied similar trophic positions and fed predominantly on descended zooplankton. We suggest that the simple structure of the Sea of Japan abyssal food web, lacking abundant deposit-feeder food chains, is the result of the young evolutionary age of this community rather than the low availability of bottom detritus or the specific structure of the pelagic community that provides abundant downward flow of zooplankton.
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